Orthogonal Frequency-Division Multiplexing (OFDM), also referred to as “multi-carrier modulation” (MCM) or “discrete multi-tone modulation” (DMTM), splits up and encodes high-speed incoming serial data, modulating it over a plurality of different carrier frequencies (called “subcarriers”) within a communication channel to transmit the data from one user to another. The serial information is broken up into a plurality of sub-signals that are transmitted simultaneously over the subcarriers in parallel.
By spacing the subcarrier frequencies at intervals of the frequency of the symbols to transmit, the peak power of each modulated subcarrier lines up exactly with zero power components of the other modulated subcarriers, thereby providing orthogonality (independence and separability of the individual subcarriers. This allows a good spectral efficiency (close to optimal) and minima inter-channel interference (ICI), i.e. interferences between the subcarriers.
For this reason, OFDM is used in many applications. Many digital transmission systems have adopted OFDM as the modulation technique such as digital broadcasting terrestrial TV (DVB-T), digital audio broadcasting (DAB), terrestrial integrated services digital broadcasting ISDB-T), digital subscriber line (xDSL), WLAN systems, e.g. based on the IEEE 802.11 a/g standards, Cable TV systems, etc.
However, the advantages of the OFDM can be useful only when the orthogonality is maintained.
To extract data from the OFDM signal, its timing should be accurately determined. Finding the symbol timing for OFDM system is nothing more than finding the beginning of the OFDM symbol. This can be achieved by finding any boundary in the preamble of the OFDM data packet. This task, implemented by OFDM receivers, is often referred to as “Symbol Boundary Detection” (SBD).
FIG. 1 illustrates such an OFDM data packet DP in accordance with IEEE 801.11 a/g WLAN standard.
The data packet DP includes a short Training Field, STF, comprising 10 identical short preambles s1, s2, s3 . . . s10. Each preamble includes 16 time samples of duration of 50 ns. The time duration of the STF preambles is therefore 0.8 μs. The Short Training Field STF has total duration of 8 μs.
The STF is followed by a Long Training Field, LTF. The LTF begins with a Guard Interval, GI2 and further comprise two identical LTF preambles, L1 and L2.
The time duration of the guard interval GI2 is 1.6 μs and it includes 32 time samples. The time duration of each LTF preambles is 3.2 μs and each of them include 64 time samples. The total duration of the LTF is 8 μs.
The LTF is followed by a SIG field which has a total duration of 4 μs. It comprises a guard interval GI and a signal field S.
The Rest of Packet field, ROP, includes data represented as symbols in which each symbol, Data1, Data2 . . . includes 64 time samples. Each symbol is preceded by a guard interval GI.
Usually, the symbol boundary detection (SBD) consists in determining the boundary between the last STF preamble s10 and the guard interval GI2, i.e. the boundary between the Short Training Field STF and the Long Training Field LTF.
Determination of the symbol boundary is very important since several OFDM mechanisms need this boundary in order to be initiated. Such mechanisms comprise for instance Carrier Frequency Offset (CFO) determination, Automatic Gain Control (AGC), Channel Estimation, Diversity selection, frame validation, etc.
As these mechanisms should be initiated as soon as possible in order to decode properly the ROP field containing the signal symbols, it is important to determine the symbol boundary as soon as possible during the reception of the training fields.
However, at the beginning of the STF, the received signal is not well conditioned, i.e. some of its parameters like its timing, its frequency, its amplitude, its phase, are not yet accurately determined. Therefore, in order to not postpone the determination of the symbol boundary any further, only a “coarse” symbol boundary determination is initiated at this stage, and a “fine” symbol boundary determination is initiated further, during reception of the LTF, to get a more accurate determination.
Several mechanisms have been proposed for coarse determination the symbol boundary, like auto-correlation on the STF.
This auto-correlation scheme exploits the periodic nature of the symbols in the STF, where a sample belonging to one preamble is repeated at the corresponding sample position in the subsequently received preambles. Therefore, the auto-correlation values rise as the receiver starts receiving the short preambles. Then, the auto-correlation values become stable for a time duration during which the STF is received, thereby forming sort of a plateau. A fall in the auto-correlation values marks the end of the STF, and the beginning of the Guard Interval GI2. The sample corresponding to this fall is recorded and used as estimation for the (coarse) symbol boundary.
However, due to high noise and low signal to noise ratio (SNR), the sampling instant corresponding to this fall may be recorded at a position that is offset from the correct boundary, due to the poor correlation metric of shot preambles at low SNR.
Accordingly, the accuracy of a coarse symbol boundary determination according to this auto-correlation scheme is poor.
However, even if only a coarse estimation is required at this step, its accuracy is important as many subsequent mechanisms are based on this first coarse estimation.
Some proposals have been made to improve the accuracy of the coarse symbol boundary determination, but they imply more complexity for their implementation.
It is however important to keep the implementations as easy as possible in order to keep low the manufacturing costs, the design costs, the silica foot-print, the energy consumptions, etc.
There is therefore a need for a solution permitting to improve the accuracy of the coarse determination of the symbol boundary, while not delaying its computing period (to avoid postponing the start of the subsequent OFDM mechanisms), and keeping the implementation simple.